This invention relates generally to inductors, and more particularly to inductors used in induction heating.
Inductors or inductor coils are generally used to heat conductive material by currents induced by varying an electromagnetic field. Electromagnetic energy is transferred from the inductor to a workpiece. For purposes of analogy, if the inductor coil is considered to be the primary winding of a transformer, then the workpiece which is about to be heated would be considered the single-turn secondary. When an alternating current flows in the primary coil or inductor, secondary currents will be induced in the workpiece. These induced currents are called eddy currents and the current flowing in the workpiece can be considered as the summation of all of the eddy currents. Heat is generated in the workpiece by hysteresis and eddy current losses, with the heat generated being a result of the energy expended in overcoming the electrical resistance of the workpiece. Typically, close spacing is used between the inductor coil and the workpiece, and high coil currents are used to obtain maximum induced eddy currents and resulting high heating rates.
Induction heating is widely employed in the metal working industry to heat metals for soldering, brazing, annealing, hardening, forging, induction melting and sintering, as well as for other various induction heating applications. As compared to other conventional processes, induction heating has several inherent advantages. First, heating is induced directly into the material. It is therefore an extremely rapid method of heating. It is not limited by the relatively slow rate of heat diffusion in conventional processes using surface contact or radiant heating methods. Second, because of a skin effect, heating is localized and the area of the workpiece to be heated is determined by the shape and size of the inductor coil. Third, induction heating is easily controllable, resulting in uniform high quality of the product. Fourth, induction heating lends itself to automation, in-line processing, and automatic process cycle control. Fifth, start-up time is short, and thus standby losses are low or nonexistent. And sixth, working conditions are better because of the absence of noise, fumes, and radiated heat. Of course, there are also other advantages.
It is well known that the magnetic flux generated by the inductor must be dense enough to bring the workpiece to a desired temperature in a specified time (typically short). When the workpiece is simple in shape and can easily be surrounded by the inductor, rapid heating using a conventional inductor is a relatively simple task. However, when the workpiece is of a more complex shape, it becomes difficult to assure rapid and uniform heating in areas which are not readily accessible to the inductor.
In the past, it has been recognized that the performance of inductors may be improved by controlling the direction of flux flow and thereby manipulating and maximizing flux density on the workpiece. For example, with an inductor coil of generally circular cross-section, directional control might be improved by attaching magnetic field orienting elements on certain portions of the circumference, so that flux is intensified on the other portion or portions. Presently used field orienting elements include laminations made of grain-oriented iron (which are relatively thin pieces of strip stock) which are attached to the inductor on a strip by strip or layer by layer basis as necessary. These laminations, however, are unsatisfactory to the extent that they are difficult to apply, requiring cutting and sizing to the necessary configuration. Thus limited inductor cross-sections are coverable because of the difficulty of application. In this regard, it is very tedious and difficult to laminate such strip stock on to complicated geometrical shapes of the type which are often needed to treat certain types of workpieces. Applying such laminations to large inductors is also somewhat prohibitive due primarily to cost and labor considerations. In addition, these iron laminations have a tendency to lose permeability at high operating temperatures. This results in inefficient heat treating operations. At high temperatures, these materials require cooling due to relatively high hysteresis and eddy current losses. Laminations made of grain-oriented iron are also relatively expensive due to the labor costs required for manufacture.
Another conventional method of controlling the direction of inductor flux density is by the use of blocks or inserts made of ferromagnetic material in a binder. Although these materials perform well, they are all prefabricated and thus are available only in a specified number of shapes and sizes. Such blocks or inserts would typically be glued to the inductor as needed to increase flux density around the insert and consequently on the workpiece. Of course, the various prefabricated sizes may also be filed, sawed, drilled, laminated to one another, or machined to unlimited numbers of sizes, but this involves a considerable amount of labor on the part of the inductor manufacturer or user. Needless to say, such labor is expensive, and this expense would be in addition to the cost of the inserts themselves, which is by no means negligible.
Accordingly, it is a principal object of the present invention to provide an improved inductor which in addition to furnishing improved directional control, does so by utilizing an easy to apply coating on the inductor. Thus a more efficient inductor may be provided which does not require extensive labor to manufacture or use.
In general, the inductor, coating, and method of the present invention are adapted to improve the directional control of an inductor by increasing magnetic flux density in only designated areas, thereby increasing and intensifying flux density on a subject workpiece being heat treated. The method of the present invention includes using a fluidized bed or other like methods to coat an inductor with a coating composition. According to the present invention, the coating composition is comprised of a low reluctance material such as carbonyl iron powder or the like, and a binder such as a polymeric resin or the like. In carrying out the method of the present invention, a conventional fluidized bed apparatus may be used to apply the coating composition to an appropriately masked inductor. After coating, the masking, which may be comprised of such materials as teflon or aluminum foil, is removed. However, prior to removing the masking, the entire inductor assembly may be coated with a protective coating such as vinyl or the like to help prevent damage to the coating composition of the present invention. In addition, it is believed that the present invention is also usable in improving the directional control of magnetic flux in other electrical conductors. As noted above, coaing methods other than fluidized bed coating are also contemplated.
Additional advantages and features of the present invention will become apparent from a reading of the detailed description of the preferred embodiments which makes reference to the following set of drawings in which: